Applications of photocatalyst techniques have come to be practically used while taking advantage of the property of accelerating various chemical reactions including the decomposition of environmental pollutants, malodorous components/various germs, etc. Examples thereof include antibacterial tiles for use in operating rooms in hospitals, filters for air cleaners and air conditioners, and glasses for, e.g., the lighting of expressways or the like. Besides such practical uses where the oxidation-accelerating ability of photocatalysts is utilized, investigations are being made for the purpose of causing a photocatalyst to act on water or the like to obtain hydrogen or to act on carbon dioxide to fix/reduce the carbon.
On the other hand, from the standpoints of the impoverishment of fossil energy resources and environmental issues such as air pollution caused by global warming, there is a desire for the establishment of a technique for obtaining a clean and safe energy and a cleaning technique for treating environmental pollutants. Of such techniques, use of a photocatalyst is promising. For example, application of a photocatalyst to the step of desulfurizing a crude oil or to a desulfurization step in metal refining may be promising.
The step of crude-oil desulfurization presently in general use is as follows. When a crude oil is distilled, the heavy naphtha is subjected to hydrofining, whereby all the sulfur ingredients contained in the crude oil are converted to hydrogen sulfide and recovered. This hydrogen sulfide is treated by the process called the Claus process and recovered through sulfur oxidation. The Claus process is a process in which one-third of the hydrogen sulfide is oxidized to sulfur dioxide and this sulfur dioxide is reacted with the remaining hydrogen sulfide to obtain elemental sulfur.
This process necessitates a huge amount of energy because heating and condensation are repeated besides the catalytic reaction of sulfur dioxide with hydrogen sulfide. It further has problems, for example, that the management of sulfur dioxide is costly. If a method which comprises adding a photocatalyst to an aqueous alkali solution containing hydrogen sulfide dissolved therein, irradiating the photocatalyst to a light to cause it to absorb the energy of the incident light and generate free electrons and free holes, and oxidizing/reducing the aqueous alkali solution containing dissolved hydrogen sulfide with the free electrons and free holes to obtain hydrogen and sulfur, i.e., a method in which hydrogen sulfide is decomposed with a photocatalyst to generate hydrogen and sulfur, can be put to practical use, then it becomes possible to decompose hydrogen sulfide, which is a hazardous substance, and produce hydrogen and sulfur, which are useful substances, using a smaller amount of energy. Namely, this contributes to the resolution of an environmental issue and enables the production of useful substances.
On the other hand, with respect to the generation of hydrogen by electrolysis, the process in which water is electrolyzed by means of the electromotive force of solar cells is being conducted. In this process, however, the efficiency of electrolysis is governed by the performance of the solar cells. There has hence been a problem that since the devices constituting high-performance solar cells are high-purity high-quality devices, such solar cells are expensive.
In this respect also, if a method in which water is decomposed with a photocatalyst to generate hydrogen can be put to practical use, it becomes possible to produce hydrogen with a smaller energy amount at a lower cost.
However, photocatalysts heretofore in use have had the following problems to be overcome. First, their catalytic activity is low. Secondly, the photocatalysts are toxic. Although photocatalysts generate free electrons and free holes upon irradiation with a light, it is highly probable that these free electrons and free holes recombine. Furthermore, there also is a high possibility that a chemical substance which has been decomposed by an oxidation/reduction reaction might undergo recombination and return to the original compound. Low catalytic activity hence results. Thirdly, the catalysts have a short life. Although the catalysts generate free electrons and free holes upon irradiation with a light, the catalysts themselves are oxidized/reduced, besides a target chemical substance, due to the strong oxidation/reduction reactions caused by the free electrons and holes. Namely, there is a problem of photodissolution that the catalysts thus dissolve away and lose their catalytic activity.
In order to overcome those problems, patent document 1 discloses a photocatalyst having high catalytic activity, no toxicity, and a long life. There is a statement therein to the effect that those three problems have been eliminated.
Also known is a method of treating hydrogen sulfide or method of producing hydrogen in which a stratified-structure electrode comprising a photocatalyst activated with a metal is used.    Patent Document 1: JP-A-2001-190964